The invention relates to fiber-reinforced ceramic composites. In particular, it also relates to axially symmetric components made of fiber-reinforced ceramic composites, in particular cylindrical disks, for example friction bodies.
Carbon fiber-reinforced composites or ceramics, in particular those having an SiC-containing matrix, are of particular interest for the production of highly tribologically stressed materials as used, for example, for friction bodies such as brake disks and clutch disks. These materials have a high thermal stability, low wear and good tribological properties.
In DE-A 199 39 545, it is proposed to increase the stressability and performance compared to the prior art by using clutch disks with friction linings of carbon fiber-reinforced silicon carbide (SiC). No details are given regarding the precise nature of the material.
A disadvantage of this material is that the production of a dense SiC matrix having carbon fiber reinforcement can be achieved only with difficulty in industry. A major improvement is infiltration of carbon fiber-reinforced carbon-containing intermediate bodies with liquid silicon to give composites containing carbon fibers as reinforcing fibers and whose matrix comprises phases of metallic silicon and silicon carbide.
DE-C 44 38 455 discloses producing friction units, in particular brake and clutch bodies, composed of C/Cxe2x80x94SiC (carbon fiber-reinforced materials whose matrix comprises silicon, silicon carbide and residual carbon) by infiltration of porous, carbon fiber-reinforced carbon bodies (C/C bodies) provided with hollow spaces and recesses with liquid silicon. However, the recesses and hollow spaces decrease the strength of the body. Studies have shown that even solid disks are unsuitable for high rotational speeds or rates of rotation of the disk, as occur, for example, in the case of clutch disks.
In EP-A 1124074 and EP-A 1124071 brake disks are disclosed which are composed of C/SiC (carbon fiber-reinforced ceramic composites whose matrix comprises silicon carbide) and methods of producing these in which, apart from a base material reinforced with bundles of short carbon fibers, long fibers are also provided for reinforcement. The long fibers extend around the shape of the brake disk. This is said to inhibit crack growth and increase the strength of the disk. The matrix material of these disks has a uniform composition, and this leads to a homogenous material composition of the finished part.
In the patent application DE 10157583.1 friction bodies are described composed of C/SiC which have a friction layer reinforced with long fibers aligned in the plane of the disk. The C/SiC surrounding the long fibers is, like the covering layer, reinforced with short fibers. The production process starts from inserts of long fiber bundles embedded in a pressable composition containing short fibers. The friction layer has a smaller proportion of short carbon fibers than the core zone.
Both of the abovementioned production methods are unsuitable for achieving high long fiber contents and a uniform long fiber distribution over the entire body. However, both are prerequisites for a further increase in strength under rotational stress.
It is therefore an object of the invention to provide fiber-reinforced ceramic composites for producing shaped bodies, in particular friction bodies, which have increased strength under rotational stress, in particular at high rates of rotation. A further object is to find a technically simple and inexpensive process for introducing the long fiber reinforcement. Another object is to provide a covering layer for these friction bodies which has been optimized in respect of the frictional and wear properties.
This object is achieved by a fiber-reinforced ceramic composite, in particular a C/SiC material, whith regions located in the interior of the shaped bodies produced therefrom reinforced with bundles, tows or hanks of fibers which are aligned essentially parallel to or concentric with the outer contour or circumference and completely surrounded by a short fiber-reinforced matrix, preferably C/SiC, and with a covering layer comprising a similar short fiber-reinforced matrix material having a lower fiber content, in particular a covering layer of C/SiC having a higher SiC content than the interior region. The reinforcing fibers, i.e. the short and long fibers, are selected independently of one another, from fibers having a mean diameter of from 4 to 12 xcexcm, preferably from 5 to 10 xcexcm and in particular from 6 to 8 xcexcm. The reinforcing fibers are preferably carbon fibers.
The invention therefore provides fiber-reinforced ceramic composites comprising bundles, tows or hanks of fibers which are surrounded completely by a short fiber-reinforced matrix, with the long and short fibers being selected, independently of one another, from fibers having a mean diameter of from 4 to 12 xcexcm.
Furthermore, the invention relates to shaped bodies comprising such materials, in particular friction bodies in the form of cylindrical disks or annular disks, which preferably also have at least one covering layer of a similar short fiber-reinforced matrix material having a lower fiber content, in particular a covering layer of C/SiC having a higher SiC content than in the interior region.
The invention likewise provides a process for producing such fiber-reinforced ceramic materials and also a process for producing shaped bodies comprising these materials.
The ceramic composites of the invention are preferably C/SiC ceramics comprising carbon fibers as reinforcing fibers and mainly phases of silicon carbide and silicon as matrix.